Multi-wavelength light source and discrete-wavelength-variable light source

ABSTRACT

In a light source for generating light containing multiple wavelengths substantially uniform in intensity, a wavelength demultiplexing element  10  (for example, waveguide-type wavelength selecting filter) demultiplexes input light into a plurality of wavelengths λ 1  through λ 32.  Optical amplifiers  14 - 1  through  14 - 32  amplify outputs of the element  10  and applies them to input ports of a wavelength multiplexing element  12.  The wavelength multiplexing element  12  wavelength-multiplexes their input. Output of the wavelength multiplexing element  12  is applied to a fiber coupler  16  which, in turn, applies one of its outputs to the wavelength demultiplexing element  10.  The optical amplifiers  14  have a gain larger by approximately 10 dB than the loss in the optical loop made of the element  10,  optical amplifier  14,  element  12  and fiber coupler  16.  The other output of the fiber coupler  16  is wavelength-multiplex light containing wavelengths λ 1  through λ 32.

FIELD OF THE INVENTION

[0001] This invention relates to a multi-wavelength light source and adiscrete-wavelength-variable light source, and more particularly, to amulti-wavelength light source for supplying one or more optical outputswith different wavelengths concurrently or selectively and adiscrete-wavelength-variable light source capable of selecting one of aplural of wavelengths, which are suitable for transmission or tests of awavelength division/multiplex transmission system.

BACKGROUND OF THE INVENTION

[0002] In wavelength division multiplex transmission systems, it isessential to reliably obtain laser lights with a number of closewavelengths. For transmission tests or tests of optical components usedin wavelength division/multiplex transmission systems, there is the needfor a laser light source highly stable in wavelengths and outputs.

[0003] ITU has recommended 0.8 nm (100 GHz) as the wavelength intervalin wavelength division multiplex transmission systems. While temperaturecoefficients of wavelength changes of semiconductor lasers areapproximately 0.1 nm/° C. That is, semiconductor lasers are verysensitive to temperature fluctuation. Therefore, it is difficult tomaintain wavelength intervals of 0.8 nm in a number of semiconductorlaser light sources over a long period. Moreover, in ordinary lasersources, injected current is used to stabilize optical outputs. Controlcurrent for stabilization of optical outputs causes changes intemperature, and it results in changes in wavelength. That is, controlof optical outputs affects wavelengths, and makes it difficult tostabilize wavelengths.

[0004] A prior proposal to cope with the problem is to connect anoptical filter and an optically amplifying element in a ring to form amulti-wavelength light source for collectively supplying multiplewavelengths. FIG. 15 is a schematic block diagram showing a priorexample A Fabry-Perot optical filter 210, erbium-doped optical fiberamplifier 212 and optical fiber coupler 214 are connected to form aring.

[0005]FIG. 16 show characteristic diagrams of the prior example of FIG.15. FIG. 16(1) shows transparent wavelength characteristics of theFabry-Perot optical filter 210, FIG. 16(2) shows amplifyingcharacteristics of the optical fiber amplifier 212, and FIG. 16(3) showsthe spectral waveform of output wavelength. The Fabry-Perot opticalfilter 210 is a kind of wavelength selecting optical filters havingwavelength transparent characteristics which permit specific wavelengthsin certain wavelength intervals called FSR (Free Spectral Range) to passthrough as shown in FIG. 16(1). Individual transparent wavelengths ofthe Fabry-Perot optical filter 210 are selected from the spontaneousemission light generated in the optical fiber amplifier 212. The outputspectral waveform coincides with that obtained by multiplying thetransparent wavelength characteristics of the optical filter 210 by theamplifying characteristics of the optical fiber amplifier 212.Theoretically, laser oscillation outputs are obtained in wavelengthswhere the gain of the optical fiber amplifier 212 surpasses the loss ofthe optical loop.

[0006] In the prior art example shown in FIG. 15, the output intensityis large near the gain center wavelength within the amplifying range ofthe optical fiber amplifier 212, where oscillation is most liable tooccur, and largely decreases in peripheral portions, as shown in FIG.16(3). That is, the prior art example cannot realize simultaneousoscillation in multiple wavelengths in substantially uniform outputlevels.

[0007] Moreover, wavelength interval in output light in the prior artexample exclusively depends on transparent characteristics of theFabry-Perot optical filter 210. When the wavelength interval is 0.8 nm(100 GHz), the wavelength interval FSR of the transparent wavelengthcharacteristics of the Fabry-Perot optical filter 210 is less than theuniform extension width of the erbium-doped optical fiber amplifier 212.Therefore, even when a plurality of oscillation wavelengths are obtainednear the gain center wavelength of the erbium-doped optical fiberamplifier 212, mode competition occurs, and results in unstable outputintensities and wavelength fluctuations of respective wavelengths.

[0008] A Fabry-Perot semiconductor lasers is a multi-wavelength lightsource, other than the fiber ring light source. However, it involvesunacceptable fluctuations in oscillation wavelengths due to modecompetition or mode hopping, and fails to uniform intensities ofrespective oscillated wavelength components.

SUMMARY OF THE INVENTION

[0009] It is therefore an object of the invention to provide amulti-wavelength light source and a discrete-wavelength-variable lightsource capable of simultaneously or selectively outputting one or morewavelengths with a uniform intensity.

[0010] Another object of the invention is to provide a multi-wavelengthlight source capable of selecting one or more wavelengths among aplurality of wavelengths.

[0011] Another object of the invention is to provide a multi-wavelengthlight source and a discrete-wavelength-variable light source immune totemperature fluctuations.

[0012] The invention uses a wavelength demultiplex/amplify/multiplexingunit for demultiplexing input light into a plurality of differentpredetermined wavelengths, optically amplifying individual wavelengths,and multiplexing the wavelengths, and connects its output to its inputto form an optical loop. Since individual wavelengths are opticallyamplified by the wavelength demultiplex/amplify/multiplexing unit, laseroscillation in a plurality of wavelengths with substantially the sameintensity is promised in the optical loop. Since the structure is simpleand the most elements are passive ones, it is highly stable againsttemperature fluctuations.

[0013] When using wavelength demultiplexing means for demultiplexinginput light into a plurality of predetermined wavelengths inpredetermined wavelength intervals, the light containing multiplewavelengths in substantially constant wavelength intervals can beobtained. Usable as the wavelength demultiplexing means is awaveguide-type wavelength selecting filter, for example.

[0014] When using the optical band pass filter means which istransparent only to light within a predetermined wavelength band, thelight source can prevent that light beyond the desired wavelength bandcirculates in the optical loop. This contributes not only tostabilization of laser oscillation but also to reliably preventing thatthe output contains undesirable wavelengths.

[0015] By using optical modulation means for intensity-modulatingcirculating light in the optical loop with a modulation signal having afrequency, which is an integer multiple of the circulation frequency(namely, c/nL) in the optical loop, the light source can conjoinmultiple-wavelength light into pulsating light synchronous with themodulation signal. Location of the optical modulation means may beeither posterior to wavelength division or posterior to wavelengthmultiplexing. When it is located after wavelength division, fineadjustment of individual wavelengths is easier, but a plurality ofoptical modulating means for individual wavelengths must be used. Whenit is located after wavelength multiplexing, optical modulating meansmay be only one, but adjustment of individual wavelengths must be donein another portion. Polarization adjusting means may be provided in theinput side of the optical modulating means to previously adjustpolarization so as to ensure optimum operations of the opticalmodulating means. If, of course, necessary means is of a polarizationholding type, polarization adjusting means may be omitted to reduceelements.

[0016] When individual optically amplifying means are capable ofselectively supplying or blocking outputs to the wavelength multiplexingmeans, multiplex output light containing one or more selectedwavelengths can be obtained. If each of the optically amplifying meanscomprises an optical amplifier for amplifying corresponding one ofoptical outputs from the wavelength demultiplexing means and an opticalswitch means for feeding or blocking the optical output of the opticalamplifier, undesired noise light is prevented from entering into thewavelength multiplexing means while the optical switch means blocks thepath.

[0017] In another aspect of the invention, an output of wavelengthdemultiplex/amplify/multiplexing means for demultiplexing input lightinto a plurality of predetermined wavelengths, then optically amplifyingthem individually, and thereafter multiplexing them is connected to theinput of the same wavelength demultiplex/amplify/multiplexing means viapolarization means, optical dividing means and depolarization means toform an optical loop. There is also provided modulation means formodulating divisional optical outputs from the optical dividing means inaccordance with a modulation signal.

[0018] With this arrangement, light components with multiple wavelengthswhich are simultaneously oscillated in the optical loop can be modulatedcollectively by the modulation means.

[0019] Since the polarization means suppresses fluctuations in plane ofpolarization, fluctuations in the ring cavity mode are less likely tooccur, and simultaneous oscillation in multiple wavelengths isstabilized. Since the polarization adjusting means in an appropriatelocation selects and maintains an appropriate plane of polarization foreach element, behaviors of individual elements are stabilized. Ifessential means are of a polarization holding type, polarization meansand polarization adjusting means may be omitted to reduce elements.

[0020] In another aspect of the invention, an output of wavelengthdemultiplex/amplify/multiplexing means for demultiplexing input lightinto a plurality of predetermined wavelengths, then optically amplifyingthem individually, and thereafter multiplexing them is connected to theinput of the same wavelength demultiplex/amplify/multiplexing means toform an optical loop and wavelength shifting means is provided in theoptical loop to slightly shift the wavelengths. As a result, laseroscillation is suppressed, and an ASE (Amplified Spontaneous emission)light source for multiple wavelengths can be realized.

[0021] Since the polarization adjusting means and depolarization meansin appropriate locations select and maintain an appropriate plane ofpolarization for each element, behaviors of individual elements arestabilized. By taking out the light from the optical loop afterdepolarization, output light independent from or less dependent onpolarization can be obtained. If essential means are of a polarizationholding type, polarization adjusting means may be omitted to reduceelements.

[0022] In another aspect of the invention, an optical loop is formedsuch as demultiplexing input light into a plurality of predeterminedwavelengths, then optically amplifying them individually, thereaftermultiplexing them and feedback to the input, and it is activated forsimultaneous oscillation in multiple wavelengths. Lights which arewavelength-demultiplexed and individually amplified are divided,individually modulated outside and thereafter wavelength-multiplexed. Asa result, multi-wavelength light containing individually modifiedwavelengths can be obtained.

[0023] In another version of the invention, an output ofselective-demultiplex/amplifying means for selectively demultiplexing apredetermined wavelength from input light and optically amplifying it isconnected to the input of the selective-demultiplex/amplifying means toform an optical loop. Thus, a single wavelength selected by theselective-demultiplex/amplifying means can be supplied as output light.That is, any one of a plurality of discrete wavelengths can be selected.Since it is selected from predetermined wavelengths, output with astable wavelength can be obtained. Since the polarization adjustingmeans in an appropriate location selects and maintains an appropriateplane of polarization for each element, behaviors of individual elementsare stabilized. If essential means are of a polarization holding type,polarization adjusting means may be omitted to reduce elements.

DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic block diagram showing a general constructionof a first embodiment of the invention;

[0025]FIG. 2 shows waveform characteristics of the same embodiment;

[0026]FIG. 3 is a schematic block diagram of a general construction of amodified embodiment;

[0027]FIG. 4 shows waveform diagrams of a version using AWGs as awavelength demultiplexing element 10 and a wavelength multiplexingelement 12, and containing two FSRs of these AWGs in the band ofamplification of the optical amplifier 14;

[0028]FIG. 5 shows a waveform obtained by an experiment;

[0029]FIG. 6 is a schematic block diagram showing a general constructionof a version commonly using two optical amplifiers 14-1 and 14-2;

[0030]FIG. 7 is a schematic block diagram showing a general constructionof an embodiment configured to modulate multi-wavelength lightcollectively;

[0031]FIG. 8 is a schematic block diagram showing a general constructionof an embodiment of the invention applied to a multi-wavelength ASElight source;

[0032]FIG. 9 is a schematic block diagram showing a general constructionof an embodiment configured to modulate each wavelength componentindividually;

[0033]FIG. 10 is a schematic block diagram showing a generalconstruction of an embodiment configured to extract one or more desiredwavelengths among a plurality of wavelengths in given wavelengthintervals;

[0034]FIG. 11 is a schematic block diagram showing a generalconstruction of an embodiment applied to a wavelength-variable lightsource for outputting a single discrete wavelength;

[0035]FIG. 12 shows distribution of wavelengths in output of theembodiment shown in FIG. 11;

[0036]FIG. 13 is a schematic block diagram showing a generalconstruction of an embodiment applied to a multi-wavelength mode lockpulse light source;

[0037]FIG. 14 is a schematic block diagram showing a generalconstruction of a wavelength converting apparatus using thewavelength-variable light source shown in FIGS. 10 and 12 as its pumplight source;

[0038]FIG. 15 is a schematic block diagram showing a generalconstruction of a conventional multi-wavelength light source; and

[0039]FIG. 16 is a characteristics diagram of the conventional lightsource shown in FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION

[0040] Embodiments of the invention are explained in detail withreference to the drawings.

[0041]FIG. 1 is a schematic block diagram showing a general constructionof a first embodiment of the invention. FIG. 2 shows wavelengthcharacteristics of this embodiment.

[0042] In FIG. 1, reference numeral 10 denotes a wavelengthdemultiplexing element for demultiplexing input light through an inputport #1 into a plurality of predetermined wavelength components (in thisembodiment, components of wavelengths λ1 to λ2). Numeral 12 denotes awavelength multiplexing element for wavelength-multiplexing the lightcomponents with multiple wavelengths (in this embodiment, wavelengths λ1to λ32). Namely, these elements are waveguide-type wavelength selectingfilters (AWG). Other than AWG, known as another optical element fordemultiplexing and multiplexing a plurality of wavelengths collectivelyis the optical demultiplex/multiplexing filter developed by OpticalCorporation of America, U.S.A. Also this type of optical element can beused as the wavelength demultiplexing element 10 and the wavelengthmultiplexing element 12.

[0043] Output ports #1 through #32 of the wavelength demultiplexingelement 10 are connected to input ports #1 through #32 of the wavelengthmultiplexing element 12 via optical amplifiers 14 (14-1 through 14-32).Output port #1 of the wavelength multiplexing element 12 is connected toa fiber coupler 16, and one of two outputs of the fiber coupler 16 isconnected to input port #1 of the wavelength demultiplexing element 10.Thus, the other output of the fiber coupler 16 is extracted as desiredmulti-wavelength light. The unused output end of the fiber coupler 16 ismade as a non-reflective end. As a result, instable oscillation byFresnel reflection can be prevented. The same also applies to allembodiments shown below.

[0044] Each optical amplifier 14 includes an erbium-doped optical fiberamplifier, pumping light source and wavelength demultiplex/multiplexing(WDM) coupler for supplying output light from the pumping light sourceto the optical fiber amplifier. The optical amplifier 14 may be made ofa semiconductor laser amplifier and a Raman amplifier.

[0045] Briefly explained below are functions of AWG used as thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12. AWG is an optical element in which wavelengths λ1 throughλ32 entering in the input port #1 are output from output ports #1through #32, and wavelengths λ1 through λ32 entering in the input port#2 are output from output ports #2 through #32 and #1. Those entering inthe subsequent ports are output from output ports with corresponding andsubsequent numbers, and wavelengths λ1 through λ32 entering in the inputport #32 are output from output ports #32 and #1 to #31. Wavelengthintervals of wavelengths λ1 through λ32 are determined by the innerinterference structure. Therefore, when wavelength-division multiplexedlight containing wavelengths λ1 through λ32 enters in the input ports#1, these wavelengths λ1 through λ32 are wavelength-demultiplexed andoutput from corresponding output ports #1 through #32. In contrast, whenlights of wavelengths λ1 through λ32 enter in respective correspondinginput ports #1 through #32, wavelength-multiplexed light containing theentered wavelengths λ1 through λ32 is output from the output port #1.

[0046] AWGs have a periodicity, and these wavelengths λ1 through λ32 areso-called basic waves. Also longer wavelengths λ1′ through λ32′ andshorter wavelengths λ31″′ and λ32″′ are wavelength-demultiplexed andwavelength-multiplexed.

[0047]FIG. 2 (1) shows composite transparent wavelength characteristicsobtained when output ports of the wavelength demultiplexing element 10are connected to common-numbered input ports of the wavelengthmultiplexing element 12, respectively. In this embodiment, in which thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12 are 32×32 type AWGs, the transparent wavelengthcharacteristics are periodic, and 32 wavelengths form one cycle, asexplained above. In general, this is defined as FSR (Free SpectralRange) of AWG. There are wavelengths λ1′, λ2′, . . . and . . . , λ31″′and λ32″′ outside λ1 through λ32 used in the embodiment, as shown inFIG. 2(1). For example, λ1′ and λ32″′ entering in the input port #1 ofthe wavelength demultiplexing element 10 are output from output ports #1and #32.

[0048] If the composite transparent wavelength characteristics of thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12 are such that the transparent wavelength width for eachwavelength is sufficiently narrow, longitudinal modes of ring resonance,described later, can be decreased to a few or only one. This is attainedby narrowing the transparent wavelength width of each wavelength in thetransparent wavelength characteristics of the wavelength demultiplexingelement 10 and the wavelength multiplexing element 12, respectively, orby slightly shifting the transparent wavelength characteristics of thewavelength demultiplexing element 10 from those of the wavelengthmultiplexing element 12. The latter is advantageous for obtaining adesired wavelength width more easily although the loss is larger.

[0049] Ideally, each of the optical amplifiers 14-1 through 14-32 hasamplifying wavelength characteristics covering one cycle of FSR, namely,the wavelength range of λ1 through λ32, and preferably exhibiting adrastic decrease in gain beyond the range. Actually, in accordance withthe amplifying wavelength characteristics of available opticalamplifiers, AWGs having FSRs consistent with the amplifying wavelengthcharacteristics are used as the wavelength demultiplexing element 10 andthe wavelength multiplexing element 12. The gain of amplification byeach optical amplifier 14 is determined larger by approximately 10 dBthan the loss in one circulation of the loop made of the wavelengthdemultiplexing g element 10, optical amplifier 14, wavelengthmultiplexing element 12 and fiber coupler 16.

[0050] Basically, it is sufficient for each of the optical amplifiers14-1 through 14-32 that its gain center wavelength can cover a singlewavelength assigned to it. However, the use of different opticalamplifiers with different gain center wavelengths makes the process ofproducing and assembling respective element more troublesome, and it ispreferable to use optical amplifiers 14-1 to 14-32 with the sameamplifying wavelength range. From this point of view, the amplifyingwavelength characteristics in which the gain is flat throughout onecycle, namely one FSR OF AWGs 10 AND 12, and drastically decreasesoutside this range is preferable.

[0051] Due to the composite transparent wavelength characteristics ofthe wavelength demultiplexing element 10 and the wavelength multiplexingelement 12 (FIG. 2(1)) and the amplifying wavelength characteristics ofthe optical amplifiers 14-1 through 14-32 (FIG. 2(2)), the loop gain ofthe embodiment shown in FIG. 1 draws peaks at wavelengths λ1, . . . λ32,and light output from the fiber coupler 16 to the exterior of theoptical loop results in the spectrum shown in FIG. 2(3). Since thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12 have the same transparent center wavelength and uniformtransmissivity to respective wavelengths, and the optical amplifiers14-1 through 14-32 have substantially the same gain, respectivewavelengths λ1 to λ32 in output light extracted by the fiber coupler 16have substantially the same optical intensity. In AWG, variance in lossamong different wavelengths upon wavelength division and wavelengthmultiplexing can be readily reduced to 3 through 4 dB or less in theprocess of fabrication, and this degree of variance can be compensatedby fine adjustment of amplification gains of respective opticalamplifiers 14-1 thorough 14-32.

[0052] Since each optical amplifier 14-1 to 14-32 amplifies a singlewavelength alone, mode competition does not occur, and stableamplification of input light is promised. Therefore, this embodiment canrealize multi-wavelength oscillation in wavelengths λ1 to λ32, and cansubstantially equalize intensities of respective wavelengths.

[0053] It is no problem, provided that one of the wavelengthdemultiplexing element 10 and the wavelength multiplexing element 12,preferably the wavelength demultiplexing element 10, does not have awavelength periodicity. However, under the conditions where thewavelength demultiplexing element 10 (and the wavelength multiplexingelement 12) has a wavelength periodicity, and FSR of the wavelengthdemultiplexing characteristics is narrower than the amplification bandof the optical amplifiers 14, which results in containing two FSRs inthe amplification band of the optical amplifiers 14, the loop gainhappens to exist also for wavelengths outside λ1 to λ32, e.g.wavelengths λ1′ and λ32″′, and possibly causes mode competition orinstable oscillation.

[0054] This can be prevented by locating an optical band pass filter inthe loop to pass wavelengths λ1 to λ32 alone. FIG. 3 is a schematicblock diagram showing a general construction of another embodiment takenfor this purpose. Reference numeral 20 denotes a wavelengthdemultiplex/amplify/multiplexing unit containing the wavelengthdemultiplexing element 10, wavelength multiplexing element 12 andoptical amplifiers 14 of FIG. 1. Numeral 22 denotes an optical band passfilter (optical BPF) transparent to wavelengths λ1 through λ32 alone inoutput light from the wavelength demultiplex/amplify/multiplexing unit20. Numeral 24 denotes a fiber coupler which demultiplexes output lightof the optical BPF 22 into two components, and supplies one to thewavelength demultiplex/amplify/multiplexing unit 20 and extracts theother as multi-wavelength output.

[0055]FIG. 4 shows waveforms of a version using AWGs as the wavelengthdemultiplexing element 10 and the wavelength multiplexing element 12,and containing two FSRs of AWGs in the amplification band of the opticalamplifiers 14. FIG. 4(1) shows transparent wavelength characteristics ofAWGs used as the wavelength demultiplexing element 10 and the wavelengthmultiplexing element 12. FIG. 4(2) shows amplification characteristicsof the optical amplifiers 14. FIG. 4(3) shows wavelength characteristicsof light passing through the optical amplifier 14-1 when the optical BPF22 is not provided. As shown, since three wavelengths λ1, λ1′ and λ1″pass through the optical amplifier 14-1 and are amplified, competitionof these wavelengths invites instable oscillation of the targetwavelength λ1.

[0056]FIG. 4(4) shows transparent characteristics of the optical BPF 22.FIG. 4(5) shows wavelength characteristics of light passing through theoptical amplifier 14-1 when the optical BPF 22 is provided. Since theoptical BPF 22 permits the basic waves (λ1 to λ32) alone to circulate inthe loop, wavelength λ1 alone, here, can enter the optical amplifier14-1 and is amplified there.

[0057] In this manner, in the embodiment shown in FIG. 3, wavelengthsother than basic waves (wavelengths λ1 through λ32) are removed by theoptical BPF 22, and do not circulate in the loop. Therefore, even if thewavelength demultiplexing characteristics of the wavelengthdemultiplexing element 10 (and the wavelength multiplexing element 12)are periodic such that it demultiplexes (or they demultiplex)wavelengths other than basic waves as well, and the optical amplifiers14 can amplify these undesired waves sufficiently, stablemulti-wavelength laser oscillation containing basic waves alone isensured.

[0058] Wavelength intervals of wavelengths contained in output lightextracted from the fiber couplers 16 and 24 are determined by wavelengthselectivities of the wavelength demultiplexing element 10 and thewavelength multiplexing element 12. It is easy to design AWGs such thatthe wavelength intervals be 100 GHz (0.8 nm) or its integer multiple.Therefore, it is sufficiently possible to realize multi-wavelengthoscillation with wavelength intervals of approximately 0.8 nm.

[0059]FIG. 5 shows waveforms confirmed by an actual experiment. Used inthe experiment are AWGs of wavelength intervals of 0.7 nm as thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12. Four ports in every other sequences are connected tocounterpart ports having common numbers via optical amplifiers. It isknown that four wavelengths in intervals of 1.4 nm are oscillatedsimultaneously in substantially the same optical intensity. The sidemode suppression ratio was as good as 35 dB, and the ratio of the signallevel to the background noise level was as good as approximately 60 dB.

[0060] A quartz AWG has a temperature coefficient of approximately 0.01nm/° C. which is smaller by one digit than that of a semiconductorlaser. Therefore, the accuracy for temperature control of two AWGs usedas the wavelength demultiplexing element 10 and the wavelengthmultiplexing element 12 can be alleviated to {fraction (1/10)} of theaccuracy required for a signal-generating semiconductor laser forgenerating signal light used in wavelength multiplexing. Consideringthat the temperature controlling accuracy of the pumping light source ofthe optical amplifiers 14 (for example, a semiconductor laser for awavelength around 1.48 nm) need not be so high as that required for asignal-generating semiconductor laser, temperature control of thepumping light source can be simplified. That is, this embodiment makesthe entire temperature control easier and simpler, and can bemanufactured economically.

[0061] This embodiment also makes it easy to adjust and modifywavelengths in output light because, by selecting appropriatetemperatures of AWGs used as the wavelength demultiplexing element 10and the wavelength multiplexing element 12, λ1 to λ32 can be shifted tolonger or shorter wavelengths while maintaining the same wavelengthintervals.

[0062] In most cases, the optical amplifiers 14 have their own pumpinglight sources. However, erbium-doped optical fibers of a plurality ofoptical amplifiers can be pumped by a single pumping light source. FIG.6 is a schematic block diagram showing a general construction of themodified part of a modified embodiment in this respect. Parts orelements common to those of FIG. 1 are labelled with common referencenumerals. Output port #1 of the wavelength demultiplexing element 10 isconnected to input port #1 of the wavelength multiplexing element 12 viaan optical isolator 30-1, erbium doped optical fiber 32-1 and wavelengthdemultiplex/multiplexing (WDM) coupler 34-1. Similarly, output port #2of the wavelength demultiplexing element 10 is connected to input port#2 of the wavelength multiplexing element 12 via an optical isolator30-2, erbium-doped optical fiber 32-2 and wavelengthdemultiplex/multiplexing coupler 34-2.

[0063] Output light of a 1.48 μm pumping semiconductor laser 36 isdivided into two parts by a 3 dB coupler 38, and one of them is suppliedto the erbium-doped optical fiber 32-1 via the WDM coupler 34-1 whilethe other is supplied to the erbium-doped optical fiber 32-2 via the WDMcoupler 34-2. The optical isolators 30-1, 30-2 prevent that pumpinglight to the erbium-doped optical fibers 32-1, 32-2 enter the outputports #1 and #2 of the wavelength demultiplexing element 10.

[0064] In this manner, the optical amplifiers 14-1 and 14-2 can share asingle pumping light source. By using this arrangement also for otheroptical amplifiers 14-3 through 14-32, the total number of pumping lightsources can be reduced to a half.

[0065] It is convenient to use light containing collectively modifiedmultiple wavelengths in transmission tests of wavelength divisionmultiplex optical transmission systems. Explained below is an embodimentin which multi-wavelength light is modified collectively. FIG. 7 is aschematic block diagram of its general construction. Numeral 40 denotesa wavelength demultiplex/amplify/multiplexing unit containing thewavelength demultiplexing element 10, optical amplifiers 14 andwavelength multiplexing element 12 of FIG. 1 (and optical BPF 22 of FIG.3). Output light of the unit 40 enters in a fiber coupler 44 via apolarizer 42. One of outputs of the fiber coupler 44 is fed to thewavelength demultiplex/amplify/multiplexing unit 40 through apolarization adjuster 46 and a depolarizer 48. The other output of thefiber coupler 44 enters into the external optical modulator 52 through apolarization adjuster 50.

[0066] In a fiber ring or loop formed by the wavelengthdemultiplex/amplify/multiplexing unit 40, polarizer 42, fiber coupler44, polarization adjuster 46 and depolarizer 48, laser oscillation ofmultiple wavelengths occur simultaneously in substantially the sameintensity in the same manner as the embodiment shown in FIG. 1. Themulti-wavelength light is extracted from the fiber ring by the fibercoupler 50.

[0067] Also in the wavelength demultiplex/amplify/multiplexing unit 40,the composite transparent wavelength characteristics of the wavelengthdemultiplexing and the wavelength multiplexing are chosen tosufficiently narrow the transparent wavelength widths for individualwavelengths so that longitudinal modes can be decreased to a few or onlyone. As explained with reference to FIG. 1, this is attained bynarrowing the transparent wavelength width of each wavelength in thetransparent wavelength characteristics of each of the wavelengthdemultiplexing element and the wavelength multiplexing element, or byslightly shifting the transparent wavelength characteristics of thewavelength demultiplexing element from those of the wavelengthmultiplexing element.

[0068] The use of the polarizer 42 contributes to suppression ofpolarization fluctuations in the fiber ring. In order to preventinterference in the wavelength demultiplex/amplify/multiplexing unit 40,the depolarizer 48 depolarizes the input light. If the polarizedcondition by the polarizer 42 is maintained, interference or otherundesirable effects may occur in the external modulator 52. To removesuch trouble in the external modulator 52, the polarization adjuster 50adjusts the polarization. Additionally, for more effectivedepolarization by the depolarizer 48, the polarization adjuster 46adjusts polarization of the input light.

[0069] While light circulates in the fiber ring made of the wavelengthdemultiplex/amplify/multiplexing unit 40, polarizer 42, fiber coupler44, polarization adjuster 46 and depolarizer 48, simultaneous laseroscillation in multiple wavelengths occurs in the same manner as theembodiment of FIG. 1. The multi-wavelength light by the simultaneouslaser oscillation is extracted by the fiber coupler 44, and applied tothe external modulator 52 via the polarization adjuster 50. The externaloptical modulator 52 modulates the applied multi-wavelength lightcollectively in accordance with an externally applied modulation signal.The modulated light is supplied to transmission optical fibers, etc.

[0070] Polarization fluctuation in the fiber ring causes fluctuation ofthe ring cavity mode, and makes simultaneous oscillation of multiplewavelengths instable. In the embodiment, however, since the polarizer 42suppresses fluctuations in planes of polarization, instable oscillationcan be suppressed. If, however, the wavelengthdemultiplex/amplify/multiplexing unit 40 (wavelength demultiplexingelement 10, wavelength multiplexing element 12 and optical amplifiers14) and the fiber coupler 16 are of a polarization holding type, thepolarizer 42, depolarizer 48 and polarization adjusters 46, 50 may beomitted.

[0071] Also in the embodiment shown in FIG. 7, if looping of light ofundesired wavelengths outside the target wavelength band should bepreviously prevented, an optical BPF similar to the optical BPF 22 inthe embodiment shown in FIG. 3 is placed at a desired location in thefiber ring (inside or outside the wavelengthdemultiplex/amplify/multiplexing unit 20).

[0072] To test characteristics of optical components, it is desirable touse an ASE (Amplified Spontaneous Emission) light source for multiplewavelengths, which does not laser-oscillate. Such a multi-wavelength ASElight source can be readily obtained according to the invention. FIG. 8is a schematic block diagram showing a general construction of anembodiment taken for this purpose.

[0073] Explained below is the construction of the embodiment of FIG. 8.Numeral 60 denotes a wavelength demultiplex/amplify/multiplexing unitsimilar to the wavelength demultiplex/amplify/multiplexing unit 40.Output light from the unit 60 enters in an acousto-optic modulator 64via a polarization adjuster 62. Output of an A/O modulator 64 enters ina fiber coupler 70 via a polarization adjuster 66 and a depolarizer 68.One of outputs of the fiber coupler 70 enters in the wavelengthdemultiplex/amplify/multiplexing unit 60, and the other output of thefiber coupler 70 is extracted as multi-wavelength ASE light.

[0074] The A/O modulator 64 slightly shifts and outputs wavelengths inthe input light. Therefore, light circulating in the fiber ring made ofthe wavelength demultiplex/amplify/multiplexing unit 60, polarizationadjuster 62, A/O modulator 64, polarization adjuster 66, depolarizer 68and fiber coupler 70 is slightly shifted in wavelength by the A/Omodulator 64. As a result, laser oscillation does not occur, andamplified spontaneous emission light, that is, ASE light is obtained.Since the multi-wavelength state is not lost even after passing the A/Omodulator 64, the light extracted from the fiber coupler 70 is ASE lightcontaining multiple wavelengths.

[0075] In order to prevent interference or other undesired events in theA/O modulator 64, the polarization adjuster adjusts polarization ofinput light to the A/O modulator 64. If the output light of the A/Omodulator 64 remains in a specific polarized state, undesirable effectsmay occur in the wavelength demultiplex/amplify/multiplexing unit 60. Todeal with the matter, the polarization adjuster 66 and depolarizer 68previously cancel the specific polarized state. The polarizationadjuster 66 and the depolarizer 68 may be located between the fibercoupler 70 and the wavelength demultiplex/amplify/multiplexing unit 60.However, as shown in FIG. 8, when they are located between the A/Omodulator 64 and the fiber coupler 70, polarization dependency isremoved from multi-wavelength ASE light extracted from the fiber coupler70, and this light can be used more conveniently for examining variouscharacteristics (such as amplification characteristics or losscharacteristics) of optical components to wavelength-divisionmultiplexed light.

[0076] In the embodiment shown in FIG. 7, multi-wavelength light ismodified collectively. However, it is preferable that individualwavelengths can be data-modulated independently for use in actualtransmission tests or transmission.

[0077]FIG. 9 is a schematic block diagram showing a general constructionof an embodiment configured to modify respective wavelengthsindividually. Numeral 80 denotes a wavelength demultiplexing elementsimilar to the wavelength demultiplexing element 10, and 82 denotes awavelength multiplexing element similar to the wavelength multiplexingelement 12. Output ports of the wavelength demultiplexing element 80 areconnected to common-numbered input ports of the wavelength multiplexingelement 82 through optical amplifiers 84 (84-1 through 84-32) similar tothe optical amplifiers 14. Wavelength multiplex output of the wavelengthmultiplexing element 82 is connected to the input of the wavelengthdemultiplexing element 80. Here again, if necessary, an optical BPFsimilar to the optical BPF 22 used in the embodiment of FIG. 3 may beprovided, for example, between the output of the wavelength multiplexingelement 82 and the input of the wavelength demultiplexing element 80.

[0078] Since this embodiment does not take out multi-wavelength lightdirectly, it does not use a fiber coupler similar to the fiber coupler16. Instead, fiber couplers 86 (86-1 through 86-32) are provided fordividing outputs of the optical amplifiers 84-1 through 84-32. Opticaloutputs extracted by the fiber couplers 86-1 to 86-32 are applied toexternal modulators 88 (88-1 through 88-32). The external modulators 88(88-1 through 88-32) are supplied with different modulation signals #1through #32. Optical outputs from the external modulators 88-1 through88-32 are applied to a wavelength multiplexing element 90 which isidentical to the wavelength multiplexing element 82.

[0079] The wavelength demultiplexing element 80, wavelength multiplexingelement 82, optical amplifiers 84 and fiber coupler 86 are of apolarization holding type. If not, additional elements corresponding tothe polarizer 42, polarization adjuster 46 and depolarizer 48 used inthe embodiment of FIG. 7 must be provided in the loop made of thewavelength demultiplexing element 80, optical amplifiers 84 andwavelength multiplexing element 82.

[0080] Composite transparent wavelength characteristics of thewavelength demultiplexing element 80 and the wavelength multiplexingelement 82 are chosen to sufficiently narrow the transparent wavelengthwidths for individual wavelengths so that longitudinal modes can bedecreased to a few or only one. As explained with reference to FIG. 1,this is attained by narrowing the transparent wavelength width for eachwavelength of the transparent wavelength characteristics of each of thewavelength demultiplexing element 80 and the wavelength multiplexingelement 82, or by slightly shifting the transparent wavelengthcharacteristics of the wavelength demultiplexing element 80 from thoseof the wavelength multiplexing element 82.

[0081] Explained below are behaviors of the embodiment shown in FIG. 9.In the loop made of the wavelength demultiplexing element 80, opticalamplifiers 84 and wavelength multiplexing element 82, laser oscillationof multiple wavelengths occur simultaneously in substantially the sameintensity in the same manner as the embodiment shown in FIG. 1.Respective wavelengths by laser oscillation are taken out individuallyby the fiber couplers 86-1 through 86-3, and applied to the externalmodulators 88-1 through 88-32. External modulators 88-1 and 88-32modulate their optical inputs by modulation signals #1 to #32,respectively. As a result, modulated optical outputs containingdifferent wavelengths modulated by different modulation signals #1 to#32 can be obtained. Then, the wavelength multiplexing element 90wavelength-multiplexes the outputs of the external modulators 88-1through 88-32, and supplies the multiplexed light to an external elementsuch as optical fiber transmission path, for example. Thus, thetransmission test can be executed in practical conditions fortransmission.

[0082] It is essential for the wavelength multiplexing element 90 onlyto compose or multiplex optical outputs of the external modulators 88-1through 88-32, and it need not have the same wavelength multiplexingfunction as that of the wavelength multiplexing element 82.

[0083] In some applications, it is desired to take out one or somewavelengths from a number of wavelengths in certain wavelengthintervals. Such requirement is attained by modifying the embodiment ofFIG. 1 in the manner as shown in FIG. 10. That is, optical switches 92(92-1 through 92-32) are inserted between outputs of optical amplifiers14-1 through 14-32 and input ports of the wavelength multiplexingelement 12. When one or more of the optical switches 92-1 through 92-32are turned on, corresponding wavelengths alone circulate in the fiberring, and laser oscillated outputs with corresponding wavelengths aretaken out from the fiber coupler 16. For example, when only the opticalswitch 92-4 is turned on, only the wavelength λ4 stimulates laseroscillation, and the laser light is taken out from the fiber coupler 16.If optical switches in every two intervals are turned on among opticalswitches 92-1 through 92-32, then multi-wavelength light containingwavelengths in wavelength interval twice that of the wavelengthdemultiplexing element 10 (and the wavelength multiplexing element 12)can be obtained.

[0084] In the same manner as the embodiment shown in FIG. 3, which is amodified version of the embodiment of FIG. 1, an optical BPF similar tothe optical BPF 22 in the embodiment of FIG. 3 may be provided, ifnecessary.

[0085] According to the embodiment shown in FIG. 10, light containingonly one or some of a plurality of predetermined wavelengths can beobtained. That is, this light source can be operated as adiscrete-wavelength-variable light source or as a multi-wavelength lightsource capable of selecting any desired wavelength interval.

[0086] The modification in the embodiment shown in FIG. 10 is applicablealso to embodiments shown in FIGS. 7, 8 and 9.

[0087]FIG. 11 is a schematic block diagram showing a generalconstruction of an embodiment realizing a wavelength-variable lightsource for a discrete single wavelength. Numeral 110 denotes awavelength demultiplexing element similar to the wavelengthdemultiplexing element 10, and 112 denotes a wavelength multiplexingelement similar to the wavelength multiplexing element 12. 114designates a 32×1 optical switch for selecting one of plural outputports (32 output ports in this embodiment) of the wavelengthdemultiplexing element 110. 116 denotes an optical amplifier foramplifying output light from the optical switch 114. 118 denotes a 1×32optical switch for switching an output of the optical amplifier 116 toone of plural input ports (32 input ports in this embodiment) of thewavelength multiplexing element 112.

[0088] Optical switches 114, 118 can be turned ON and OFF by using acommon switching signal. That is, optical switches 114, 118 select anoutput port and an input port with a common number among plural outputports of the wavelength demultiplexing element 110 and plural inputports of the wavelength multiplexing element 112.

[0089] Since the optical amplifier 116 amplifies one of wavelengths λ1through λ32 demultiplexed by the wavelength demultiplexing element 110,its amplification band is wide enough to cover wavelengths λ1 throughλ32 and need not be wider. No problem of FSR occurs.

[0090] Explained below are behaviors of the embodiment shown in FIG. 11.Among wavelengths λ1 through λ32 demultiplexed by the wavelengthdemultiplexing element 110, a wavelength selected by the optical switch114 is amplified by the optical amplifier 116. Output of the opticalamplifier 116 enters in one of input ports of the wavelengthmultiplexing element 112, having a number common to the output portselected by the optical switch 114. Therefore, the wavelengthmultiplexing element 112 outputs light amplified by the opticalamplifier 116 from its output port to the fiber coupler 120. The fibercoupler 120 divides the light from the wavelength multiplexing element112 into two components, and supplies one to the wavelengthdemultiplexing element 110 and externally outputs the other as outputlight.

[0091] The light of the wavelength selected by the optical switches 114,118 circulates in the fiber ring made of the wavelength demultiplexingelement 110, optical switch 114, optical amplifier 116, optical switch118, wavelength multiplexing element 112 and fiber coupler 120, andstimulates laser oscillation.

[0092]FIG. 12 shows an example of wavelength distribution in output ofthe embodiment shown in FIG. 11. In this example, an output port #i ofthe wavelength demultiplexing element 110 and an input port #i of thewavelength multiplexing element 112, which correspond to wavelength λi,are selected by the optical switches 114, 118. In FIG. 12, the actuallylaser-oscillated wavelength is shown by the bold solid line, andwavelengths that can be selected are shown by the thin solid line.

[0093] If a sufficient wavelength selectivity is ensured only with thewavelength demultiplexing element 110, the system may omit thewavelength multiplexing element 112 and hence the optical switch 118.

[0094]FIG. 13 is a schematic block diagram showing a generalconstruction of a multi-wavelength mode-locked pulse light source takenas another embodiment of the invention. In a pulse light source, it isdesirable that the pulse phase is stable on the time domain. In thisembodiment, mode-locked pulse light for plural wavelengths can beobtained collectively.

[0095] Numeral 130 denotes a wavelength demultiplex/amplify/multiplexingunit comprising the wavelength demultiplexing element 10, opticalamplifiers 14 and wavelength multiplexing element 12, all of FIG. 1,wavelength demultiplexing element 10, optical amplifiers 14-1 through14-32, optical switches 92-1 through 92-32 and wavelength multiplexingelement 12, all of FIG. 10, or wavelength demultiplexing element 110,optical switch 114, optical amplifier 116, optical switch 118 andwavelength multiplexing element 112, all of FIG. 11. When the wavelengthdemultiplex/amplify/multiplexing unit 120 comprises the wavelengthdemultiplexing element 10, optical amplifiers 14 and wavelengthmultiplexing element 12 of FIG. 1, laser oscillation occurssimultaneously in multiple wavelengths. When the unit 120 comprises thewavelength demultiplexing element 10, optical amplifiers 14-1 through14-32, optical switches 92-1 through 92-32 and wavelength multiplexingelement 12 of FIG. 10, or the wavelength demultiplexing element 110,optical switch 114, optical amplifier 116, optical switch 118 andwavelength multiplexing element 112, laser oscillation occurs inselected one or some wavelengths.

[0096] Numeral 134 denotes an electroabsorption optical modulator formodulating output light of the wavelengthdemultiplex/amplify/multiplexing unit by a sinusoidal modulation signal,and 136 denotes a fiber coupler for dividing output light of theelectroabsorption optical modulator 134 into two parts to supply one tothe wavelength demultiplex/amplify/multiplexing unit 130 and toexternally output the other as output light.

[0097] When L is the ring length of the ring or loop made of thewavelength demultiplex/amplify/multiplexing unit 130, electroabsorptionoptical modulator 134, and fiber coupler 136, n is the effectiverefractive index, and c is the light velocity, a sinusoidal voltage of afrequency corresponding to an integer multiple of the basic frequencyfo=c/(nL) is applied as a modulation signal to the electroabsorptionoptical modulator 134. Transparent band widths of the wavelengthdemultiplexing element (and wavelength multiplexing element) in thewavelength demultiplex/amplify/multiplexing unit 130 for respectivewavelengths are determined to be sufficiently narrower than thecirculating basic frequency fo.

[0098] Under these conditions of frequency, light circulating in thefiber ring or loop made of the wavelengthdemultiplex/amplify/multiplexing unit 130, electroabsorption opticalmodulator 134 and fiber coupler 136 is mode-locked to the sinusoidalmodification signal applied to the electroabsorption optical modulator134, and has the form of a pulse rising at an apex or nadir of thesinusoidal modulation signal. As a result, a sequence of pulsescontaining multiple wavelengths and mode-locked can be obtained.

[0099] Since the ring length L and the effective refractive index n varyfor different wavelengths, it is necessary, in a strict sense, to adjusteffective optical path lengths for individual wavelengths in thewavelength demultiplex/amplify/multiplexing unit 130. However, it issufficient to connect an electroabsorption optical modulator (and, ifnecessarily, a polarization adjuster) in a location anterior to theoptical path for each wavelength, more preferably, anterior to theoptical amplifier 14, in the wavelength demultiplex/amplify/multiplexingunit 130 and to apply a sinusoidal modulation signal in correspondingphase and frequency to the electroabsorption optical modulator tomodulate it there. Then, the phase and frequency of one sinusoidalsignal may be adjusted independently, and may be applied as a modulationsignal to each electroabsorption optical modulator. In this case,however, A number of electroabsorption optical modulators (andpolarization adjusters) corresponding to respective wavelengths areneeded and the light source becomes more expensive than the embodimentof FIG. 13.

[0100] In the embodiment of FIG. 13, when looping of undesiredwavelengths other than the target wavelengths should be prevented, anoptical BPF similar to the optical BPF 22 in the embodiment shown inFIG. 3 is provided at an appropriate location, for example, between theoutput of the wavelength demultiplex/amplify/multiplexing unit 130 andthe optical modulator 134.

[0101] By using the wavelength variable light source according to theembodiment shown in FIG. 10 or FIG. 11 as a pump light source of awavelength converting apparatus, any wavelength acceptable in a networkcan be used efficiently. FIG. 14 is a schematic block diagram of ageneral construction of an embodiment taken for this purpose.

[0102] In FIG. 14, numeral 140 denotes a wavelength variable lightsource shown in FIG. 10 or FIG. 11, which is designed and fabricated sothat wavelengths acceptable in a wavelength-division multiplexingoptical network can be selected. Output light from the wavelengthvariable light source is applied as pumping light λp to a semiconductorlaser amplifier 142. On the other hand, input modified light λs entersinto a terminal A of an optical circulator 144. The optical circulator144 is an optical element which outputs the light entering through theterminal A from another terminal B and outputs the light enteringthrough the terminal B from a terminal C. Output light from the terminalB of the optical circulator 144 (modulated light λs) is fed to an endsurface of the semiconductor laser amplifier 142 opposite from the endsurface into which the pumping light λp is entered.

[0103] The pumping light λp and the modulated light λs travel inopposite direction within the semiconductor laser amplifier 142. If theintensity of the pump light λp is held at a value where the gain of thesemiconductor laser amplifier 142 is saturated, the pumping light λp iswaveform-modified in accordance with the intensity waveform of themodulated light λs due to their mutual gain modulation effect. That is,waveform of the pumping light λp becomes substantially opposite from thewaveform of the modulated light λs. The waveform-modified pumping lightλp enters into the optical circulator 144 through the terminal B, and itis output from the terminal C. The light output from the terminal C ofthe optical circulator 144 has a form in which the input modulated lightλs has been wavelength-converted to the wavelength of the pump light λp.

[0104] In the discrete-wavelength-variable light source 140 to which theinvention is applied, its available wavelengths can be readily set tocoincide with wavelengths acceptable in the wavelength-divisionmultiplexing optical network. Once the wavelengths are set so, thewavelength of the optical signal obtained by wavelength conversion ofthe input modified signal λs is an acceptable wavelength of the network,and the acceptable wavelength in the network can be re-used. When alight source capable of varying continuous wavelengths, such asconventional multi-electrode semiconductor laser, for example, is usedas the wavelength variable light source 140, precisely accurate controlmust be made to adjust the wavelength of its output light to one ofwavelengths acceptable in the network, and this invites a muchcomplicated construction and a high cost. In contrast, according to theinvention, the discrete-wavelength-variable light source can select anappropriate wavelength through the switch, and can remove the need forwavelength control and severe accuracy therefor.

[0105] In addition to the foregoing examples, there are arrangements forfour-wave-mixing, for example, as wavelength converting mechanisms, anda fiber amplifier is also usable in lieu of the semiconductor laseramplifier and the arrangement using an absorption-type optical modulatoris disclosed in a patent application by the same Applicant, entitledWaveform Converting Apparatus, (Japanese Patent Application Heisei8-233796).

[0106] As readily understandable from the above explanation, accordingto the invention, laser output containing multiple wavelengths withsubstantially uniform intensity can be obtained. By using as wavelengthdemultiplexing means an element for wavelength-demultiplexing inputlight into multiple wavelengths in predetermined wavelength intervals, amulti-wavelength light source for generating light containing multiplewavelengths in certain intervals can be realized. Since the light sourcehas a simple structure and is mostly of passive elements, it is stableagainst changes in temperature.

[0107] By intensity-modifying circulating light in an optical loop witha modification signal having an integer multiple frequency of thecircular frequency of the optical loop, multi-wavelength pulse lightlocked with the modulation signal can be obtained.

[0108] By locating means posterior to wavelength division (preferably,posterior to optical amplification) for selectively supplying light toor blocking light from wavelength multiplexing means, multiplex outputlight containing any selected one or more wavelengths can be obtained.

[0109] According to the invention, it is also easy to modifymulti-wavelength laser light either collectively or individually.

[0110] When wavelength shifting means is placed in an optical loop, anASE light source for multiple wavelengths can be realized.

[0111] When the output of selectively demultiplexing and amplifyingmeans for selectively demultiplexing a predetermined wavelength frominput light and amplifying the demultiplexed light is connected to inputof the same means so as to form an optical loop, one of a plurality ofdiscrete wavelengths can be used as output light. That is, one ofdiscrete wavelengths can be selected. Since the wavelength is selectedfrom predetermined wavelengths, output containing stable wavelengths canbe obtained.

1. A multi-wavelength light source for outputting laser light containingdifferent wavelengths simultaneously, comprising: wavelengthdemultiplexing means for demultiplexing input light into a plurality ofpredetermined different wavelengths; a plurality of optically amplifyingmeans for amplifying said predetermined different wavelengths from saidwavelength demultiplexing means individually; wavelength multiplexingmeans for wavelength-multiplexing optical outputs of respective saidoptically amplifying means; connecting means for connecting an output ofsaid wavelength multiplexing means to an input of said wavelengthdemultiplexing means; and output take-out means for taking out lightcirculating in a loop comprising of said wavelength demultiplexingmeans, said optically amplifying means, said wavelength multiplexingmeans and said connecting mean to the exterior of said loop.
 2. Themulti-wavelength light source according to claim 1 wherein saidwavelength demultiplexing means demultiplexes said input light into aplurality of predetermined different wavelengths in predeterminedwavelength intervals.
 3. The multi-wavelength light source according toclaim 2 wherein said wavelength demultiplexing means is a waveguide-typewavelength selecting filter.
 4. The multi-wavelength light sourceaccording to claim 1 wherein said wavelength multiplexing meanswavelength-multiplexes optical inputs having predetermined wavelengthswhich enter in respective said inputs.
 5. The multi-wavelength lightsource according to claim 4 wherein said wavelength multiplexing meansis a waveguide-type wavelength selecting filter.
 6. The multi-wavelengthlight source according to claim 1 wherein said output take-out means islight splitting means for taking out light travelling through saidconnecting means.
 7. The multi-wavelength light source according toclaim 1 further comprising optical band pass filter means provided onsaid connecting means and transparent only to light within apredetermined wavelength band.
 8. The multi-wavelength light sourceaccording to claim 1 further comprising optical modulating meansprovided on said optical loop for intensity-modifying circulating lightin said optical loop in accordance with a modification signal, saidmodification signal having a frequency corresponding to an integermultiple of the circular frequency of said optical loop, and said lighttaken out by said output take-out means is pulsating light.
 9. Themulti-wavelength light source according to claim 8 wherein said means onsaid optical loop is of a polarization holding type.
 10. Themulti-wavelength light source according to claim 8 further comprisingpolarization adjusting means located in an input side of said opticalmodulating means.
 11. The multi-wavelength light source according toclaim 1 wherein each of said optically amplifying means can selectivelysupply or block its output to said wavelength multiplexing means. 12.The multi-wavelength light source according to claim 10 wherein each ofsaid optically amplifying means includes an optical amplifier foramplifying corresponding one of optical outputs from said wavelengthdemultiplexing means and optical switching means for supplying orblocking an output of said optical amplifier.
 13. A multi-wavelengthlight source for outputting laser light containing a plurality ofcollectively modified wavelengths, comprising: wavelengthdemultiplex/amplify/multiplexing means for demultiplexing input lightinto a plurality of predetermined different wavelengths, then opticallyamplifying them individually, and wavelength-multiplexing them;polarizing means for extracting predetermined polarized components fromoutput light of said wavelength demultiplex/amplify/multiplexing means;light splitting means for splitting output light of said polarizingmeans; depolarizing means for depolarizing one of optical outputs ofsaid light splitting means and for applying it to wavelengthdemultiplex/amplify/multiplexing means; and modulating means formodulating the other optical output of said splitting means inaccordance with a modulation signal.
 14. The multi-wavelength lightsource according to claim 13 further comprising first polarizationadjusting means located between one of outputs of said splitting meansand input of said depolarizing means.
 15. The multi-wavelength lightsource according to claim 13 further comprising second polarizationadjusting means located between the other output of said splitting meansand input of said modulating means.
 16. The multi-wavelength lightsource according to claim 13 wherein said wavelengthdemultiplex/amplify/multiplexing means comprises wavelengthdemultiplexing means for demultiplexing input light into predetermineddifferent wavelengths in predetermined wavelength intervals, a pluralityof optically amplifying means for amplifying said wavelengthsdemultiplexed by said wavelength demultiplexing means individually, andwavelength multiplexing means for wavelength-multiplexing opticaloutputs from respective said optically amplifying means.
 17. Themulti-wavelength light source according to claim 16 wherein saidwavelength demultiplexing means and said wavelength multiplexing meansare waveguide-type wavelength selecting filters.
 18. A multi-wavelengthlight source for outputting ASE light containing a plurality ofwavelengths, comprising: wavelength demultiplex/amplify/multiplexingmeans for demultiplexing input light into a plurality of predetermineddifferent wavelengths, then optically amplifying them individually, andwavelength-multiplexing them; wavelength shifting means for slightlyshifting wavelengths in output light from said wavelengthdemultiplex/amplify/multiplexing means and for returning it back toinput of said wavelength demultiplex/amplify/multiplexing means; andoutput take-out means for taking out light which is circulated by saidwavelength demultiplex/amplify/multiplexing means and said wavelengthshifting means.
 19. The multi-wavelength light source according to claim18 wherein said wavelength demultiplex/amplify/multiplexing means andsaid output take-out means are of a polarization holding type.
 20. Themulti-wavelength light source according to claim 18 further comprisingfirst polarization adjusting means for adjusting polarization of outputlight from said wavelength demultiplex/amplify/multiplexing means andfor supplying it to said wavelength shifting means.
 21. Themulti-wavelength light source according to claim 18 further comprisingdepolarizing means for depolarizing output light from said wavelengthshifting means and for supplying it to said wavelengthdemultiplex/amplify/multiplexing means.
 22. The multi-wavelength lightsource according to claim 21 wherein said depolarizing means comprisessecond polarization adjusting means for adjusting polarization of outputlight from said wavelength shifting means, and a depolarizing elementfor depolarizing output light from said second polarization adjustingmeans.
 23. The multi-wavelength light source according to claim 21wherein said output take-out means is located between output of saiddepolarizing means and input of said wavelengthdemultiplex/amplify/multiplexing means.
 24. The multi-wavelength lightsource according to claim 18 wherein said wavelength shifting means isan acousto-optic modulator.
 25. The multi-wavelength light sourceaccording to claim 18 wherein said wavelengthdemultiplex/amplify/multiplexing means comprises wavelengthdemultiplexing means for demultiplexing input light into predetermineddifferent wavelengths in predetermined wavelength intervals, a pluralityof optically amplifying means for amplifying said wavelengthsdemultiplexed by said wavelength demultiplexing means individually, andwavelength multiplexing means for wavelength-multiplexing opticaloutputs from respective said optically amplifying means.
 26. Themulti-wavelength light source according to claim 25 wherein saidwavelength demultiplexing means and said wavelength multiplexing meansare waveguide-type wavelength selecting filters.
 27. A multi-wavelengthlight source for outputting light containing a plurality of individuallymodified wavelengths, comprising: wavelength demultiplexing means fordemultiplexing input light into a plurality of different wavelengths; aplurality of optically amplifying means each for amplifying one of saiddifferent wavelengths from said wavelength demultiplexing meansindividually; a plurality of splitting means each for splitting opticaloutput from associated one of said optically amplifying means; firstwavelength multiplexing means for wavelength-multiplexing first opticaloutputs of said splitting means and supplying the multiplexed output tosaid wavelength demultiplexing means; a plurality of optical modulatingmeans capable of modulating second optical output of said splittingmeans individually; and second wavelength multiplexing means forwavelength-multiplexing outputs of said optical modulating means. 28.The multi-wavelength light source according to claim 27 wherein saidwavelength demultiplexing means demultiplexes said input light intodifferent predetermined wavelengths in predetermined wavelengthintervals.
 29. The multi-wavelength light source according to claim 27wherein said wavelength demultiplexing means is a waveguide-typewavelength selective filter.
 30. The multi-wavelength light sourceaccording to claim 27 wherein said first and second wavelengthdemultiplexing means wavelength-multiplex optical inputs entering in itsinputs and having predetermined wavelengths.
 31. The multi-wavelengthlight source according to claim 30 wherein said first and secondwavelength multiplexing means are waveguide-type wavelength selectingfilters.
 32. A discrete-wavelength-variable light source for selectivelysupplying one or more of discrete wavelengths contained in output laserlight, comprising: selective demultiplex/amplifying means forselectively demultiplexing one or more predetermined wavelengths frominput light and optically amplifying them; and optical splitting meansfor part of optical output from said selective demultiplex/amplifyingmeans to an input of said selective demultiplex/amplifying means and forexternally supplying the remainder of said optical output from saidselective demultiplex/amplifying means.
 33. Thediscrete-wavelength-variable light source according to claim 31 whereinsaid selective demultiplex/amplifying means comprises wavelengthdemultiplexing means for demultiplexing input light into a plurality ofpredetermined wavelengths, first optical switch means for selecting oneof said wavelength components from said wavelength demultiplexing means,and optically amplifying means for amplifying optical output of saidfirst optical switch means.
 34. The discrete-wavelength-variable lightsource according to claim 33 wherein said selectivedemultiplex/amplifying means further comprises wavelength multiplexingmeans for wavelength-multiplexing a plurality of optical inputs withwavelength multiplexing characteristics consistent with input ports, andsecond optical switch means for supplying optical output of saidoptically amplifying means to one of input ports of said wavelengthmultiplexing means corresponding to the wavelength selected by saidfirst optical switch means.
 35. The discrete-wavelength-variable lightsource according to claim 35 wherein said wavelength demultiplexingmeans demultiplexes said input light into predetermined wavelengths inpredetermined wavelength intervals.
 36. The discrete-wavelength-variablelight source according to claim 35 wherein said wavelengthdemultiplexing means is a waveguide-type wavelength selecting filter.37. The discrete-wavelength-variable light source according to claim 34wherein said wavelength multiplexing means is a waveguide-typewavelength selecting filter.
 38. The discrete-wavelength-variable lightsource according to claim 32 further comprising optical modulating meansprovided on an optical loop made of said selectivedemultiplex/amplifying means and said optical splitting means tointensity-modulate circulating light in said optical loop in accordancewith a modulation signal, said modulation signal having a frequencywhich is an integer multiple of a frequency circulating in said opticalloop, said remainder of said optical output from said optical splittingmeans being pulsating light.
 39. The discrete-wavelength-variable lightsource according to claim 38 wherein said selectivedemultiplex/amplifying means and said optical splitting means are of apolarization holding type.
 40. The discrete-wavelength-variable lightsource according to claim 38 further comprising polarization adjustingmeans located in an input side of said optical modulating means.